Variable-speed, constant-frequency (VSCF) power generating systems utilize a brushless, synchronous generator driven by a variable-speed prime mover. The generator produces variable-frequency AC power which is rectified and supplied over a DC link to a DC/AC converter that converts same into constant-frequency AC power. Often, the level of power required to be handled by the DC/AC converter is such as to necessitate the use of parallel connected inverters therein. In this case, it may be necessary or desirable to interconnect corresponding phase outputs of the inverter using interphase transformers that balance the currents supplied by the inverters.
When variable, unbalanced and reactive three-phase loads are connected to the inverters, harmonic distortion can become quite significant, particularly between phase outputs. If no compensation is provided for these harmonics, the distortion can rise above acceptable levels.
It has been recognized that the phase displacement between parallel connected inverter outputs affects the harmonics produced thereby. For example, Lipman, U.S. Pat. No. 4,204,264 discloses a multi-bridge three-phase converter wherein the phase displacements between parallel connected converter outputs are maintained at fixed, predetermined values to cancel one or more harmonics in the overall converter output.
Heinrich, U.S. Pat. No. 4,849,870 discloses a method of operating an AC drive having parallel DC link power converters wherein the phase shift between corresponding inverter outputs is made equal to a predetermined, fixed value, such as 30.degree., to reduce the magnitudes of certain harmonics in the inverter output. Heinrich specifically discloses at column 6, lines 18-23 that the phase shift and magnitude of power converter channels may be controlled independently. However, no disclosure or suggestion is made as to how or by what means this could be accomplished.
Several patents disclose controls for multiple inverters connected by interphase transformers. These patents include Urish, U.S. Pat. No. 3,657,633, Ellert, U.S. Pat. No. 3,883,792, Heintze, U.S. Pat. No. 3,943,429, Zach, et al., U.S. Pat. No. 4,387,421, Muto, et al., U.S. Pat. No. 4,480,299, Honbu, U.S. Pat. No. 4,549,258, Shekhawat, et al., U.S. Pat. No. 4,635,177 (assigned to the assignee of the instant application), Sekino, et al., U.S. Pat. No. 4,646,221, Severinsky, U.S. Pat. No. 4,685,042, Takahashi, U.S. Pat. No. 4,800,478 and Mizoguchi, U.S. Pat. No. 4,802,079.
None of the systems disclosed in the foregoing patents attempts to regulate or control the magnitude of harmonics in the output of parallel connected inverters in a closed loop fashion to account for varying load conditions.